Effect of novel NOP receptor ligands on food intake in rats

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand for the NOP opioid receptor, stimulates feeding in rats. The present study evaluated the effect of three newly synthesized NOP receptor agonists and two NOP receptor antagonist on food intake. Freely feeding rats were tested with intracerebroventricular (ICV) injections of the NOP receptor agonists OS-500, OS-462 and OS-461. OS-500 and OS-462 evoked a hyperphagic effect more potent and far more pronounced than that of N/OFQ, while OS-461 was ineffective. OS-500 and OS-462 were also tested by intraperitoneal injection, but were unable to evoke hyperphagia following this route of administration. The NOP receptor antagonist NC-797 and UFP-101 did not modify feeding in freely feeding rats while fully antagonized the hyperphagic effect of N/OFQ. Pre-treatment with UFP-101 but not with NC-797 antagonized the hyperphagic effect of OS-462 and OS-500. The present findings indicate that OS-500, OS-462 may act as potent and long-lasting NOP receptor agonists, whereas UFP-101 and NC-797 show antagonistic properties. The higher efficacy of UFP-101 in blocking the hyperphagic effect of OS-462 and OS-500 may be linked to the better pharmacokinetic profile of this antagonist compared to NC-797. Overall, the results indicate that these compounds may represent valuable pharmacological tools to investigate the role of the brain N/OFQ system.